The invention is directed to a method for use in communication transmission over directional radio paths, in particular, a radio-relay network.
The term "network" is here used in its accepted sense in the communication field with respect to long-distance telephone dialing networks, etc. although in the present instance common-channel radio operation is involved, i.e. microwave transmission. As is well known in the communication field, a communication network may extend between a plurality of cities, centers or points, each of which is, for example, connected with each of the others of the network. Where telephone lines are involved, each line carries transmission in both directions. However where radio transmission is involved, each line would be replaced by two radio transmissions, each comprising its respective transmitter and receiver whereby one transmitter and receiver is employed for one transmission direction and a similar receiver and transmitter employed for the opposite direction. Consequently as between two such "network points," each point would have a transmitter and a receiver. If one such network point is also connected with a plurality of other points, it would have a receiver and a transmitter for each of such additional points connected therewith, and the respective communication directions or paths from such other points to the common point thus would "meet" thereat, which common point thus can be termed a meeting or "junction" point. Obviously, the communication lines extending in different directions from such a junction point to the other points connected thereto would of necessity diverge from such junction point.
As is well known, so-called "common-channel" operation may be advantageously employed in microwave radio communication networks as the microwaves may be transmitted as a focused beam, with an angle of divergence of about 1.degree. commonly being employed. As these waves travel in a straight line, sight differences are normally involved, generally relay or repeater stations being required, usually about thirty miles apart, in dependence on the height of the antennas. Likewise, such waves are subject to rain attenuation.
In determining the operational parameters of the transmitters of a radio-relay network, one of the principal factors taken into account is the possibility of fading, i.e. the parameters involved are so dimensioned or determined that even if fading should occur, the communication transmission will still be achieved. For example, where the transmission of intelligence is in the form of digital signals, such as pulse-code-modulated signals, transmitted over directional radio systems, a wider frequency band is utilized than with an analog transmission of the same information. This, however, is offset by the advantage of digital transmission with respect to a decreased sensitivity to interferences. Thus, with a pulse-code-modulated transmission, interference such as a radio frequency interference signal or thermic noise will not cause an unpermissible error signal rate when the interference level at the receiver input is 10 to 20 decibels below the useful received signal level. However, if thermic noise and radio frequency interference signals superimpose as in the case of fading, a material greater decoupling is required between the useful signal and the radio frequency interference signal or the system will fail with only a small amount of fading. As a sufficient decoupling is often not achievable, the thermal noise must be reduced to smaller values than would be required in the absence of rf-interferences. This usually is so effected that the transmitters of the directional radio system are operated with a much higher transmitting power that would be required, in fade-free periods, for faultless transmission. Thus in turn results that during common channel operation interferences occur in the network which limit the multiple utilization of a channel (common-channel operation) and thus lower the frequency band utilization.
The present invention thus is directed to the provision of a method for communication transmission over directional radio paths in which, during common-channel operation, interference in the network is avoided to a high degree and at the same time adequate quality of the communication transmission is achieved during periods of fading.